In the case of a three-phase motor, there is a possibility that the motor can rotate even if one power line in one phase of the power lines in three phases enters the broken state. This is also true in the case where a power element of a motor power conversion device enters the open state and is destroyed and power is not supplied to one phase. In the case such as this, the motor is not controlled normally, and therefore, there is a case where a high voltage is applied to the motor or a case where large current flows, and there is a possibility that the motor may fail.
The power line of a general motor is not provided with a sensor for detecting a breakage and generally, the presence/absence of a breakage of the power line is determined indirectly by detecting whether or not current flows through the power line in the case where a voltage is applied to the power line.
For a motor control device that performs PWM control, as one of the methods for detecting the presence/absence of a breakage of the power line of a motor, there is known a method for analyzing current that flows through the power line when turning ON/OFF a switching device such as an IGBT (Insulated Gate Bipolar Transistor) so as to apply a predetermined voltage to each phase (e.g., Publication of Japanese Unexamined Patent Application No. 2009-50059 A).
However, with this method, it is necessary to perform the detection operation after temporarily turning OFF the current control, and therefore, it is not possible to detect a breakage while controlling a motor in an excited state. In other words, with this method, it is not possible to detect a breakage in the case where the power line enters the broken state during the control of the motor.
Because of this, there is known a method for determining that the power line is broken in the case where a deviation becomes one in a certain degree by monitoring the deviation between a current command value and the value of current that flows through the motor (e.g., Publication of Japanese Unexamined Patent Application No. 2004-320945 A)
In the prior art, the current deviation is monitored as it is, and therefore, such problems as follows occur.
(1) There is a delay in the current control loop, and therefore, at the instant that the current command changes, the current deviation becomes an amount corresponding to the change in the current command. Consequently, if the current deviation is monitored as it is, there is a strong possibility that erroneous detection will occur.
(2) Due to factors such as noise that has piled up on the current value, even if the power line is connected normally, there is a possibility that the current deviation will be large and erroneous detection will occur as a result.
Further, the method does not take into consideration the case where the current deviation becomes large when the voltage command value, which is the output of the current control loop, reaches the upper limit value, i.e., when the voltage command value saturates, and therefore, if this method is used for monitoring at all times, there may be a possibility that a breakage of the power line is detected erroneously.